This invention relates to a powder metallurgy method for making a low thermal expansion, thermally conductive member and to an article or member made by the method, and in particular to such a process and article wherein the thermal expansion characteristic of a composite metal powder is closely matched to that of a low expansion material such as a ceramic or other low-electrical-conductivity material over a broad temperature range including the metal-to-ceramic bonding temperature.
Electronic packages such as microelectronic circuits and power semiconductor devices are sensitive to elevated temperatures during operation. The failure rate of such devices increases dramatically with increasing operating temperature. Accordingly, such devices are usually equipped with a member used to conduct heat away from the device, i.e., a heat sink. The heat sink is usually bonded to a support member of the device. Here and throughout this application the term "support member" is defined to mean any type of substrate or mounting base for an electronic package or device, including but not limited to ceramic substrates and printed circuit boards. The support member is often made of a ceramic material, such as alumina (Al.sub.2 O.sub.3) or beryllia (BeO). The bonding techniques used include soldering or brazing. When a heat sink is bonded to a ceramic substrate by soldering or brazing, the temperature of the assembly may reach 800.degree. C. or higher.
Among the performance characteristics desired in a heat sink are (a) good thermal conductivity to provide efficient heat transfer and (b) a thermal expansion characteristic that matches the thermal expansion characteristic of the ceramic support closely enough to minimize stresses caused by any expansion mismatch during the bonding process and during operation of the electronic package. Materials such as copper, silver, or aluminum, which have good thermal and electrical conductivity also have relatively high thermal expansion characteristics that result in undesirable expansion mismatch with the known ceramic support materials. Conversely, controlled thermal expansion materials such as "Kovar".RTM. or Invar provide a good thermal expansion match with a ceramic substrate, but do not provide the high thermal conductivity of copper, silver, or aluminum.
In response to the need for a good combination of thermal conductivity and thermal expansion matching, composite materials containing both a high thermal conductivity component and a low thermal expansion component have been developed hitherto. Such composite materials include metal laminates, metal powder composites, and metal infiltrated composite materials. Each type of known composite material has some drawback which limits its desirability.
For example, metal laminates which consist of a layer of low thermal expansion material sandwiched between two layers of a high thermal conductivity material provide a good thermal expansion match with a ceramic substrate and good thermal conductivity parallel to the plane of the layers. However, such laminates have less than desirable heat transfer capability in the direction perpendicular to the plane of the layers because of the low thermal conductivity of the center layer.
Metal-infiltrated components are prepared by compacting tungsten or molybdenum powder, sintering the compact to a predetermined porosity, and then infiltrating the porous, sintered compact with molten copper or silver. While such composites provide a good combination of thermal conductivity and thermal expansion matching with ceramic substrates, the manufacturing technique is difficult to control and leaves much to be desired.
U.S. Pat. No. 4,158,719, owned by Carpenter Technology Corporation, assignee of the present application, relates to a composite metal powder article that provides low thermal expansion and good thermal conductivity. The composite article is formed of a first metal powder having an average thermal expansion coefficient over the range 25.degree. C. to 400.degree. C. of less than about 12.times.10.sup.-6 /.degree. C., intermixed with a second metal powder having a thermal conductivity greater than that of the first metal powder. A shaped article is formed by compacting and sintering the metal powder mixture.